improve the design further.

compare testStringByMovingLeft to testAppendToStringByMovingLeft
use stringByByMoving methods, not mutable string append methods to grow the string.

compare testGrowingStringAroundTapPositionCombinesLeftAndRightOld to testGrowingStringCombines
for improvement. although this improvement is laregly due to the mock object framework used. stubs on passing in a mutable string, then appending the part needed to them in an anDo block would be better.